Consider a communication network where a number of Access Nodes (ANds) are deployed in a given area. Mobile User Nodes (UNds) access the network wirelessly. In order to efficiently coordinate the network and the mobility of UNds in a densely deployed topology of ANds, e.g. situated in the range of 30-50 m apart, Coordinator Nodes (CNds) are introduced at the level of the access network domain as a control plane entity. Each CNd is responsible for a subset of ANds. The role of the CNd is to control and coordinate the communication (e.g. schedule resources, assign power levels, etc.) between UNds and the ANds within its area.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) a similar role of the CNd is handled by the Mobility Management Entity (MME), but only at a core network level. The MME handles all LTE-related control plane signalling, including mobility and security functions. In particular, the MME is responsible for idle mode UE (User Equipment) mobility including support for Tracking Area management, paging, and tagging procedure related to retransmissions. The MME is also involved in the bearer activation/deactivation procedures and is also responsible for choosing the Serving Gateway (S-GW) and the Packet Data Network Gateway (P-GW) for a UE at the initial attach and at the time of intra-LTE handover involving a Core Network (CN) node relocation.
In LTE, the role of the aforesaid CNd is borne by the Mobility Management Entity (MME). The MME change and (re-)selection process in LTE is required for different mobility and location management procedures (e.g. S1-based handover). When moving, the UE detects and measures reference signals from surrounding access nodes (eNBs in LTE nomenclature), either regularly or when instructed by the network. The UE reports the measurements back to the network side. If the Reference Symbol Received Power (RSRP) from a neighbouring eNB is larger than the serving eNB, the network (through the source eNB) initiates a mobility management procedure that can involve the MME relocation with the necessary signalling information exchange between all related network entities.
In Wideband Code Division Multiple Access (WCDMA), the corresponding equivalent of the aforesaid CNd would be the Radio Network Controller (RNC). The procedure for Inter-RNC handover (either soft or hard) also involves the measurement by the UE of pilot Ec/I0 values from different NodeBs. NodeB is the WCDMA terminology for ANd. Then, if certain threshold-based events occur, measurements are reported back to the serving NodeB and from there to the serving RNC (SRNC) which takes decisions on the handover process (e.g. add, remove or replace links in the active set of the UE). A relocation of SRNC will happen when, for example, if all radio links in the active set belong to the Drift RNC (DRNC). Thus, at some point the SNRC decides to transfer the UE context to the DRNC which will now become the new SRNC. The new SRNC will then establish a connection of the Iu interface towards the Core Network.
With the conventionally known solutions in LTE, it is always the UE that initiates mobility management procedures (e.g. related to the handover process) by reporting measurements back to the network which will then send signalling messages via the MME and may include change of MME. There will be also a response signalling back and forth between the source and target eNBs to acknowledge whether such handover is accepted or not. This procedure will be time-consuming especially when targeting for shorter latency requirements for 5G networks. In addition, link failure points in the LTE handover process arise from the fact that the UE is heavily involved in signalling between the network and itself. This is especially true when dealing with handovers in a dense small cell scenario. In this case, by the time the UE measures the corresponding neighbour's RSRP and detects a predefined offset between serving and target cells (known as A3 offset) during some specified time interval known as Time-To-Trigger (TTT), the UE may already be in a state where the link to the serving eNB is too poor to convey the measurement report. Thus a handover failure occurs. In LTE macro cell scenarios this could be partially solved by optimization of aforesaid parameters TTT and A3 offset (among others). For a dense small cell scenario this has proven to be insufficient. A solution to simplify the procedure, by making the mobile terminal agnostic to the handover process and not requiring performing network measurements, is required. This will in turn shorten the overall latency and prohibit handover failures between eNBs and MMEs.
In the case of WCDMA, measurements are also performed at the UE and are sent back to the NodeB and from there to the serving RNC. The UE synchronises to all cells that are within detection range, identifies each cell and decodes the System Frame Number (SFN). According to whether the detected cell is an already identified cell or a new cell in the neighbour list, the WCDMA standard specifies that the UE needs to report the measurements within between 200 ms and 800 ms respectively. In the considered case of dense small cell networks these measurement reporting times are insufficient. In addition, the change of serving RNC will also be triggered by such measurement reports which, with the specified timeframes, may not be enough to the reacting times required by ultra-dense networks.
Thus, in order to enable implementation of ultra-dense networks, new solutions are required for making intra control node handover of UNds.